well as far as I understood.. they were going to smuggle 600 gold (worth 6000 profit), but since Lemario couldn't get the full sum of money, and thus could buy only 100 (worth 1000 profit) he saw that the profit wouldn't be enough so they decided to trick Lawrence..
As to why Lawrence lied to Nora - maybe not to discourage her and test her faithfulness?
when striking the end deal with Lemario, who knew that there was only 1000 worth of gold, lawrence asked for half of it (500 but payed in 10 years time) to be assigned to Lawrence's guild, leaving the rest to reestablish Lemarios company.
The giuld payed lawrence some money for this deal, thus allowing him to repay all his depts..
that's about it I guess, at least how I understood..